Chilled beam technology is often misunderstood as unsuitable for hot and humid climates. However, with careful design and control, chilled beams are a viable alternative to traditional HVAC systems in these environments. This article provides an overview of chilled beam applications with methods to ensure their efficient and effective operation. It will also discuss potential first-cost and energy savings, as well as strategies for optimizing system performance.
Concepts and Benefits
Chilled beam systems use water as well as air to transport thermal energy throughout a building. The chilled beam includes a hydronic coil which provides sensible heating or cooling to the space. Either 2-pipe or 4-pipe designs are available. The benefit of the 4-pipe configuration is that some zones can receive cold water for space cooling while other zones simultaneously receive hot water for space heating.
Chilled beams come in both active and passive configurations. Both require the building ventilation and latent loads be decoupled and addressed separately. This is typically accomplished by a dedicated outdoor air unit (DOAS) which dehumidifies the required quantity of ventilation (primary) air. This dry, conditioned air is then provided to the space to handle these loads, as well as offsetting some of the space sensible load.
When conditioned primary air is supplied directly to the chilled beam itself, the device is called an “active” chilled beam (ACB). The primary air travels through nozzles in the beam where its velocity is increased, inducing additional room air through the beams coil. This induced air mixes with the primary air and is discharged back into the space through slots along the beam.
Passive chilled beams (PCB) are not directly supplied with primary air and rely completely on natural convection to provide their sensible capacity. They will not be specifically addressed in this article as they have limited application in hot and humid climates, although they can be used when needed to supplement a load requirement where an active beam falls short.
Chilled beams are ideal for applications with high internal sensible cooling loads and should be installed where the tightness of the building envelope is adequate to prevent excessive moisture transfer from outdoors. Space moisture gains due to occupancy and/or processes should also be moderate.
Successful installations of chilled beam systems have included the following applications, regardless of local climate:
- K-12 and post-secondary educational facilities;
- Office buildings;
- Data centers;
- Sensible load-driven laboratories;
- Hospital patient rooms;
- Retrofit of existing "Induction Unit" installations (circa 1950-1970).
Chilled beams are considered to be a “decoupled” system design because, as mentioned, the hydronic-based chilled beam devices are integrated with a separate DOAS unit which provides conditioned primary ventilation air to the beams. With this design concept, the following opportunities for improvements in first cost, energy consumption, space control, and maintenance may be achieved:
Energy Efficiency
Water’s superior heat transport properties allow chilled beam systems to use less energy than traditional HVACD systems. Active chilled beam systems may require up to 60%-70% less primary airflow than conventional variable air volume (VAV) systems, leading to significant fan energy savings. With chilled water temperatures typically between 58°F-60°F (14°C-16°C), the chilled water system benefits from reduced compressor lift, increased chiller capacity, and greater efficiency (increased operational hours) of any water-side economizers.
Indoor Air Quality
Air delivered through the beam at ratios of 1-part primary and 2-4 parts (induced) room air increase air-change rates and promotes mixing of the space and ventilation air. Chilled beam systems can be supplied with 100% outdoor air (to satisfy ventilation requirements) which could then be directly exhausted, eliminating pollutants that would otherwise be transported through return ductwork or between air distribution zones. This may lead to improved IAQ and thermal comfort.
Acoustics
As chilled beams use no internal fans; they minimize noise generation compared to traditional systems. With proper design, sound levels can easily be kept below 40 NC (Noise Criterion).
Reduced First-Cost and Space Requirements
Chilled beams require less air transport infrastructure, reducing overall HVACD system size and cost. This compact design can also reduce interstitial space height and floor-to-floor dimensions, while reducing building material use. Alternatively, finished ceiling heights could be increased for aesthetic purposes.
Maintenance Cost Savings
Chilled beams are sensible cooling devices and when controlled properly will not promote the formation of liquid condensate which can lead to dripping, bacterial, and mold growth. They do not require drain pans or condensates lines eliminating the need for periodic cleaning (although some manufacturers do offer a drip-tray option). They do not contain fans or filters to maintain and require only simple periodic service including occasional vacuuming of the dry hydronic coil.
Considerations in Hot and Humid Climates
Humidity becomes an issue if the surface temperature of any chilled beam cooling coil, unit panel, or exposed chilled water pipe drops below the surrounding air’s local dew-point temperature. When this occurs, there is a certainty of condensation forming.
This is addressed by the primary air system which supplies dry air to the beam to handle the internal space latent and external ventilation loads, while limiting the indoor dew-point temperature, typically below 55°F. Chilled water, supplied to the beam to handle space sensible loads, should be provided at temperatures above the surrounding beam dew-point so as not to promote the formation of condensate.
Chilled water temperatures are typically delivered at 58°F-60°F and when properly controlled will keep beam surface temperatures elevated above the local dew-point without incidence of condensation. Some designers will ensure the space dew-point temperature always remains at least 5°F-7°F below the coldest temperature of any system components surface.
Penn State Prof. Stan Mumma, PhD., P.E., and Fellow ASHRAE (2002), performed various studies on chilled ceiling panels (the chilled beam’s predecessor), with findings that concluded the formation of condensation in environments with chilled ceilings is a slow process and one that can be avoided by sound design and control.
The Role of Mechanical Dehumidification
Precise space dehumidification is crucial in ensuring the proper operation of chilled beams in hot and humid climates. Incorporating properly sized direct expansion (DX), chilled water, or solid desiccant dehumidifiers into the dedicated outdoor air system (DOAS) can effectively reduce the space’s latent load by providing exceptionally dry air.
This minimizes condensation risk on chilled beams, especially in overhead applications where moisture buildup can lead to significant operational issues, including water dripping from the beams into the space below.
Control Strategies
Primary air flow can be regulated by a volume flow limiter (VFL), which ensures the correct air volume reaches each beam. For room temperature control, chilled water flow is adjusted based on thermostat signals.
Modulating the chilled water flow typically produces a 7°F-8°F (4°C-5°C) variation in air temperature, which is often sufficient for most spaces, except those with large, fluctuating loads.
In situations where space humidity rises above design parameters, adjusting chilled water supply temperature or reducing flow to the beam can help mitigate the risk of condensation. However, these strategies should be used cautiously in humid climates to avoid thermal discomfort. As long as the space dew point temperature can be maintained within a reasonable range (+/- 2°F) and the chilled water supply temperature is at (or above) this value, condensation will be avoided on chilled beam surfaces.
Preventing the formation of condensate on chilled beam surfaces must be addressed, especially in hot and humid climates. Proper system design combined with measurement and control of space humidity will help ensure satisfactory performance. The following will discuss a few of the more common control strategies used to help make sure requirements are met.
Monitor Local Temperature and Dewpoint: The use of a high-quality space mounted temperature and relative humidity sensor can provide great results. It is important to measure humidity where the risk of condensation is highest. Responsiveness to local humidity spikes is a key requirement that goes into the selection and placement of these sensors. It is also advisable to provide additional dew point or humidity monitoring throughout the building in case any local sensor fails. All sensors must remain easily accessible and be recalibrated on occasion.
Monitor Moisture on CHWS Pipe: A sensor can be installed which will read the coldest chilled water piping location in the zone. This allows for the detection of condensation directly at its “worst case” location. When moisture is detected the zone water flow can be shut off and will not be restored until the moisture has been evaporated. This can also signal an increase in primary airflow to assist in returning the space to acceptable humidity conditions.
Monitor Outdoor Conditions: In certain situations, it may be acceptable to base control actions on indoor and outdoor air temperature and humidity. Knowing these values, along with the latent load in the building (when infiltration is predictable) allows the BAS to calculate the condensate potential.
This is not an option recommended in hot and humid climates as primary control is not based on actual space measurements.
Monitoring & Measurement
Maintaining local dew-point temperature may be the single most crucial factor in ensuring trouble free operation. Facility managers need to know at a glance whether or not they are maintaining critical space conditions in their buildings. By programming space sensor readings into a building automation system, a facility manager can easily generate a “response for action” alarm should program setpoints be out of compliance.
It is vital to be proactive to potential humidity issues which will result in the loss of space control should an unacceptable condition be detected. Immediate action must be taken in order to remedy the situation before it becomes a problem.
Implementation: The key to a successful chilled beam installation starts with a design that provides fulfillment of the buildings heating and cooling needs. Detailed analysis of sensible and (particularly) latent loads tied to internal sources, ventilation, and infiltration must result in equipment and transport systems that are sized and installed to handle loads so proper environmental set points can be maintained.
The BAS system and control strategy implemented must result in a system that will achieve the HVAC design intent. Measurement and verification will ensure trouble free operation and that the building achieves optimum environmental comfort and energy performance.
Chilled beams may require slightly more upfront design consideration than other more traditional HVAC systems, but as long as important details are not overlooked, they can be successfully applied in any climate. Superior performance comes from combining the experience and expertise of all parties involved in the building’s design, operation, and maintenance.
References:
- TROX Chilled Beam Design Guide, TB012309 (2000);
- Chilled ceilings, addressing the concerns of condensation, Mumma, SA 5. ASHRAE IAQ Applications/Fall 2003;
- Price Engineers HVAC Handbook (2002);
- DADANCO, Breathing Life into Your Buildings, CM10 (2020).
About the author
David Schurk is a senior manager at Innovative Air Technologies in Covington, GA, and an ASHRAE Distinguished Lecturer and Instructor for their Humidity Control I & II professional development training courses. He can be reached at 920-530-7677 or [email protected]